Lately, secondary batteries such as lithium-ion secondary batteries, nickel-hydrogen batteries and the like have become increasingly important as power supplies loaded on electrically-driven vehicles, or as power sources installed in PCs, mobile phones and other electric products, etc. In particular, there are expectations for the preferable use of lightweight, high energy density lithium-ion secondary batteries and other non-aqueous electrolyte secondary batteries as high-power batteries installed in vehicles. Patent Document 1 can be cited as an article disclosing the use of a positive electrode material layer that exhibits a certain pore size distribution curve in a positive electrode of such a non-aqueous electrolyte secondary battery.
When a non-aqueous electrolyte secondary battery as described above is subjected to charging, for instance, if the battery being charged has a defect, or if the charger fails and malfunctions, the battery may be supplied with an abnormally high amount of current, resulting in an overcharged state along with some defects. Thus, prevention of such defects calls for the use of a battery provided with a device (a current interrupt device, CID) that detects overcharged states by the internal temperature, pressure or the like, and interrupts current when an overcharged state is detected. In a secondary battery provided with such a CID, it has been a practice to include in the electrolyte solution an overcharge additive such as cyclohexylbenzene (CHB), biphenyl (BP), etc., having an oxidation potential lower than the non-aqueous solvent of the electrolyte solution. When the battery reaches an overcharged state, before the electrolyte solution decomposes, the overcharge additive undergoes a reaction to form gas. This is utilized to raise the degree of increase or rate of increase in the battery's internal pressure when an overcharged state is reached, thereby to activate the CID at a suitable timing to prevent occurrence of defects caused by overcharging.
Overcharge additives such as CHB, BP and the like can be used for other purposes besides as gas generators in CID-installed secondary batteries as described above. For instance, in an overcharged state, the overcharge additive undergoes a reaction to generate gas while undergoing self-polymerization. A polymer formed by this serves as a resistor in the battery. Taking advantage of this, the polymer formed from the overcharge additive is allowed to precipitate out on the positive electrode surface to form a membrane, whereby further overcharging is prevented and the safety in an overcharged state is increased.